1. Field
The present invention relates to a handler for sorting burn-in tested semiconductor chips, and more particularly to a tray-transferring unit for such a handler.
2. Background
Subsequent to wafer fabrication processing, a series of tests are typically formed on semiconductor chips to verify that they perform to specifications. An electrical test checks the general performance of the packaged chip and ensures that it meets certain input and output voltage, capacitance, and current specifications. A functional test actually exercises the specific chip functions. Burn-in tests are intended to stress the electrical interconnections of the chip and package and drive any contaminants in the body of the chip into the active circuitry, thus potentially causing failure. The early failures are detected by conducting the burn-in tests. The packaged chips that pass the burn-in test are statistically more reliable.
The burn-in test requires that the chips be mounted in rows and columns on a burn-in board. The burn-in board is then mounted in a test chamber with temperature-cycling capability. During the burn-in test, the circuits used and the chips are temperature cycled while under an electrical bias. After the burn-in testing is performed, the chips are sorted depending on whether they passed the test or failed the test.
A handler is typically used to mount chips on the burn-in board prior to testing, and to remove the tested chips from the burn-in board and to sort them after the burn-in tests have been conducted. The handler includes a main body, and a loading unit which is provided on one side of the main body. An unloading unit is used to place the tested chips in trays. The handler may further include a sorting unit which receives chips that have completely failed the tests, or chips that are suspect and require further testing.
In operation, the handler transfers burn-in tested packaged chips from a burn-in board to an empty tray waiting in the unloading unit. Simultaneously, chips which are to be subjected to the burn-in test are transferred from trays waiting in the loading unit onto the burn-in board. When a tray waiting in the loading unit is emptied, it is transferred to the unloading unit so that chips being removed from the burn-in board after testing can be re-loaded back onto the tray.
However, due to a variety of potential errors, sometimes not all chips are removed from a tray at the loading unit before the tray is transferred to the unloading unit. This could occur, for instance, if a loading picker fails to properly grasp and remove one or more chips on a tray at the loading unit. If this happens, when chips are subsequently unloaded onto the tray at the unloading unit, packaged chips from the burn-in board may be placed on top of a chip still remaining in the tray.